Liquid pulp screen



July 14, 1936.

E. J. TRIMBEY 2,047,808

LIQUID PULP SCREEN Filed Feb. 6, 1935 P x5 J r1 -I l? 17A-fusi /scf//Mnf l /f *j zfifzM/W #vga vses

Patented July 14, 1936 UNITED STATES PATENT OFFICE LIQUID PULP SCREEN Y Edward 1.-'rnmbey, Glens Falls, N.'Y. Application February s, 1935, serial No. 5,225

f zclaims. (oise- 35) This invention pertains to methods and apparatus for screening masses of solids in liquid vehicles. It has prospective utility in a variety of industrial arts; but the particular machine herein described is designed especially for the screening of fibres in the manufacture of pulp and paper. These, ordinarily, are wood fibres-either ground wood or sulphite, and, in the liquid state,

are commonly known 'as liquid pulp or pulp stock. i

More specifically, the invention has to do with a machine for performing the coarse screening yoperation on liquid pulp,"the purpose being to remove knots, slivers and any other llarge particles which may be present in the stock. Such machines are known in the pulp and paper industry as "knotters. y

'Ihe principal object of the-invention is to effect an increase of screening capacity per unit of screen area and to bring about a complete separation of good pulp from the refuse while at the same time providing a machine which can be depended upon for continuous operation and i which ischaracterized by simplicity of design and ample accessibility without being materially more costly4 tol build than competitive machines of equal size, quality and weight.

' My-new screen or knotter is oi the horizontal rotary` cylinder outflow type and, in addition to simplicity of form and ruggedness of construction, is characterized by three distinctive novel features, each of which contributes. conjointly with the others, to amarked improvement in over-all operating eiliciency-including a substantial increase in production capacity and in the thoroughness of separation of the goodf pulp from the refuse. v

One of these features is a new arangement and method of delivering the incoming unscreened liquid pulp stock to the surface of the screen whereby a large part of the stock is caused to pass through immediately-which is one of the factors contributing to the increased screening capacity of the machine. This is accomplished by the provision of a special multiple-discharge intake fitting by virtue of which the stock is caused to be directed to areas on the interior screen surface at either side of the axis of `rotation and well above the bottom of the cylinder so that the unscreened stock ows downwardly along the perforated screen surface and, for that reason, has an increased opportunity to pass through the screen plate at the very outset-as compared with the case of the stock being dumped directly into the bottom ofthe cylinder.V

,The second distinctive'feature of the invention I with the familiar revolving vring the knots and other refuse A(with adhering consists in the provision oi' means for maintaining a reservoir or pond of screened liquid pulp underneath and in contact with the bottom of the cylinderthe latter being constantly immersed in the pond. The function of vthis pro- 5 vision is to prevent excessive thickening oi the stock inside the cylinder and consequentfoullng of the screen. The pond serves to restrict the hydraulic pressure acting outwardly on. the screenmaintaining the pressure at` or near 9.110 value at which maximum screening is attainable and at the same time rendering a large part of the screen area effective at all times., Sincea large part of the total screen area is operated at or near maximum effectiveness, it follows that the output of the machine is, as a matter of course, comparatively large.

A third feature lies in the design of the discharge-end portion of the screen cylinder-the heretofore commonly employed auxiliary or separate finishing screen being eliminated, together scoops for transferpulp) to the finishing cylinder. Instead, the machine of the present invention employs a screen cylinder the discharge-end portion of which is conically tapered down to form-a' discharge port for the refuse matter of a diameter considerably smaller than that of the main portion ofthe cylinder and is provided with an vinternal helix which is operable to move therefuse matter toward the discharge port. y'I'his novel arrange- 1 ment not only possesses the advantage of greater simplicity of design; but it renders 'the' screen cylinder accessible for internal observation and makes possible the provision of an internall l shower for washing the refuse matter and thereby effecting a thorough salvaging of the good pulp stock which might otherwise adhere tothe discharged refuse and be lost.

A knetter embodying the several above-mentioned novel features is fully described hereinafter and illustrated in the accompanying drawing, wherein:

Fig. 1 is a longitudinal section in the vertical 45 plane;

Fig. 2 is a section taken on the plane of the line B-B of Fig. 1, and- Fig. 3 is an elevational view of the refusedischarge end of the machine.

I'he machine includes a horizontal, cylindrical outer shell or housing I, serving as an enclosure and supporting medium for the screen cylinder 2 which is carried by and rotatable with a, horizontal drive-shaft 3 which, during operation, is 55 continuously rotated-usually at a. speed of about thirty revolutions per minute. A driving pulley 4 is shown Amounted on the drive-shaft at the intake end of the machine.

The screen cylinder structure includes three bronze spiders 5, 6 and 1, keyed to the shaft and providing a supporting framework for the perforated screen plates which form the cylindrical screen surface and the conical screen surface. The screen plates are secured to four circumferentially spaced bronze T-bars 8, 9, I0 and II which are attached to the rims of the spiders and extend horizontally. At the intake end of the screen cylinder there is provided a bronze annular disc I2 which is secured at its periphery to the four T-bars and is rotatable with the cylinder. The disc servesas a closure element for the cylinder.

The discharge-end portion of the screen cylinder is conically tapered down to form at its extremity a circular discharge port of considerably smaller diameter than that of the main portion of the cylinder-the reason for which will presently be made clear.

Extendingaround and along the interior peripheral surface of the screen cylinder is a metal ribbon-like helix I3 the function of which is to 1effect a positive movement of the unscreened stock together with the knots and other refuse matter toward the discharge end of the machine.

Mountedunderneath the screen cylinder is a curved plate I4 forming a reservoir into which the screened liquid pulp stock flows as it comes through the screen above. This reservoir is always full of screened liquid pulp and it constitutes the ,pond", heretofore mentioned, in which the bottom lof the screen cylinder is continuously partially immersed. The overflow of the pond passes out through the screened pulp discharge opening I5.`

Unscreened liquid pulp is admitted to the interior of the continuously rotating screen cylinder by way of an intake pipe I6, elbow I'I and a two-way branched intake fitting I8 which, as will be seen from an inspection of Fig. 2, is designed to discharge the incoming stock against diametrically opposite interior screen areas at about the'level of the axis of rotation. The liquid stock flows downwardly along the screen surface, a part reaching the bottom and the remainder passing through the screen. It will be noted that the end of the helix I3 at the left as viewed in Fig. l is spaced from the inlet end of the screen and that the branched intake fitting is disposed between said end of the helix and the intake end of the screen. The helix, therefore, does not interfere with the free flow of the liquid through the screen. The initial screening thus effected is proportionately large and this accounts in substantial measure for the increased capacity of the machine.

Liquid pulp stock is admitted (if available in sufficient quantity) at a rate such as to maintain a level inside the cylinder which will result in a realization of approximately maximum output without loss of good pulp stock. If the liquid level inside the cylinder is too high theA stock either will be emitted` in part, through the refuse discharge port or (if the level is not quite high enough for the stock to be so emitted) the refuse will not be adequately washed and a substantial amount of good pulp will be lost on that account. On the other hand, if the liquid level is kept too low the output of the machine will be substantially less than its capacity. The approximately correct level of the liquid pulp in the cylinder for maximum output with minimum loss is illustrated in Fig. 2. The level to be maintained is not highly critical but may vary within limits which are practicable in mill operation. Nevertheless it is incumbent upon the machine tender to see to it that the liquid level stays within bounds--assuming that there is enough stock available to feed the machine at a rate which will enable keeping the level up.

It goes without saying, that the larger the screen area covered by the liquid pulp the greater will be the output of the machine; but this holds true only upon condition that the screen perforations are kept open. There is nothing to be gained by increasing effective screen area if by so doing the screen becomes fouled and its effectiveness thus greatly diminished. And that is precisely what would happen as a result of maintaining a high liquid level ln the machine here described were it not for the presence of the pond in which the screen cylinder is immersed. The rate of flow of liquid through a screen varies with pressure-which, of course, is proportional to the hydraulic head. Raising the liquid level in the screen cylinder increases the effective screen area and at the same time the hydraulic head. If a liquid without solid content were being dealt with, both of these factors (screen area and hydraulic head or pressure) would contribute to greater output in direct proportion to their magnitude; but it happens in thc case of liquid pulp that increasing the head above a certain allowable value tends to cause a separation of the pulp fibres from the liquid vehicle on the intake side of the screen plate with the detrimental result that the screen perforations become fouled I by the pulp. v f

Up to a certain point the increased pressure and effective screen area brought about by raising the level of the liquid pulpwill more than offset the tendency toward diminished output due to fouling of the screen; but a point of equilibrium is soon reached beyond which any further raising of the liquid level causes a reduction of output.

The effect of the pond is to offset the'fouling i' v effect of increased head by lequalizing, in part,

the hydraulic pressure inside and'outside the v screen, res activelythus controlling the rapidity of flow while permitting the liquid level to risethereby taking advantage of greater effective screen area while getting rid of what would otherwise be the concomitant offensive factor, excessive hydraulic head.

Obviously, the eifective hydraulic head is equal to the difference between the liquid level inside the screen and the overflow level of the pond. The machine should be so proportioned and the overflow level of the pond fixed at a point such that the effective head will not be too great so long as the liquid level within the cylinder does not exceed what will permit the refuse matter properly to be cleaned before being dischargeda matter which presently will be taken up.

Were it not for other factors of a contnolling character .maximum screen output would doubtless be realized by operating with the liquid pulp at the level of the axis of rotation and raising the overflow level of the pond so as to keep the difference of pressure at an appropriate value; but that is not practicable in a machine for coarse screening wood pulp because the volume of refuse matter is such as to require substantially continuous removal.

' out of the liquid mass and as it moves toward the discharge port it is washed by a. shower of water owing downwardly from shower pipe i9 which is shown projecting into the conical portion of the cylinder-see Fig, 1. The shower removes the adheringl pulp and carries it through the screen plate and the washed refuse matter passes on through the discharge port and outl into a receptacle provided for the purpose. Obviously, if the liquid level is too high-even though ,not quite high enough for the liquid pulp to overow through the refuse discharge port, the knots and other refuse will not be adequately freed of good pulp before they are discharged. The liquid level, therefore, must be kept low enough to permit the refuse being properly cleaned, but, on the other hand, high enough to realize something like maximum output-assuming that there is enough volume of incoming stock to demand efficient operation-which is usually the case.

By reason oi' the absence of radial arms within the conical portion of the screen cylinder, the shower pipe I9 can be located inside the screen instead of outside and above and this greatly increases the thoroughness with which the refuse matter is cleaned and, of course, resul-ts in a substantial saving of good pulp which would otherwise be lost.

As will be self-evident, the large refuse discharge port makes it easy to inspect the interior of the screen cylinder during operation; and, while obstructions are not likely to occur, it facilitates the removal of any objects, such as digester bricks, which may possibly find their way into the cylinder and not pass through of their own accord.

I have built and `successfully operated a machine 'in accordance with the accompanying drawing wherein the diameter of the screen cylinder was approximately thirty-nine inches and in which the length of the cylindrical portion was forty-seven inches and the conical portion twenty-two inches. From the foregoing specication oi' dimensions and the fact that the drawing was made to scale it will be possible for anyone skilled in the art to duplicate my machine and realize successful results. 'I'he perforations of the screen plates may be of the same order as `to size and spacing as are commonly employed in machinesof this class.

What is claimed is:

l. In a. machine for screening masses of solids in liquid vehicles, a screen of the rotary cylindrical outflow type having a horizontal axis of rotation, one end-portion of said screen being conically tapered to form at its extremity a refuse discharge port of considerably smaller diameter than that of the main portionof the screen, means for delivering unscreened liquid stock to the interior of the screen at the end thereof remote from said discharge port, a reservoir disposed underneath and adjacent the bottom of said screen and arranged to maintain a pond of screened liquid stock in which the bottom portion of the screen is partially immersed, and a shower pipe located within the conically tapered portion 20 of said screen and arranged to direct a shower against the interior screen surfaceof said conically tapered portion for cleaning the refuse matter prior to discharge thereof through said port.

2. In a machine for screening masses of solids in liquid vehicles, a screen of the rotary cylindrical outow type having a horizontal axis of rotation, one end-portion of said screen being conically tapered to form at its extremity a 30 refuse discharge port of considerably smaller ldiameter than that of the main portion of the screen, means for delivering unscreened liquid stock to the interior of the screen at the end thereof remote from said discharge port, said 35 means including 'a multiple-branched conduit for directing the incoming liquid stock against the interior surface of the screen at both sides of the axis of rotation thereof and at elevations well above the bottom of the screen, a reservoir disposed underneath and adjacent the bottom of said screen and arranged to maintain a pond of screened liquid stock in which the bottom portion of the screen is partially immersed, a helix disposed within and extending along the 1nterior periphery of the conically tapered portion of the screen, said helix being operable to convey solid refuse matter from the liquid mass in the bottom of the screen to the refuse discharge port, and a shower pipe located within the conicallv `tapered portion of said screen and arranged to direct a shower against the interior screen surface of said conically tapered portion.

- EDWARD J. TRIMBEY. 

